Adding to the ventilation hypothesis. The following is a look downwards from the bathroom window.

Here is what I see:

• The pipe on the right is gas.
• Looking down, I see that all bathrooms have their individual ventilation, correct? So they would not have the same problem.
• The pipe on the left appears to emerge from my bathroom. Is that the ventilation pipe headed to the roof? I’ll go there now to check it out.

https://abload.de/img/87922e5a-5f57-4ac5-bqce47.jpeg

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Update

This is the exterior view of the bathroom (the left window). It looks to me as if the pipe on the right hand side of that window emerges from that bathroom, then moves up and ends in an inverted U.

The pipe on the left that also crosses horizontally is gas and can be ignored.

All bathrooms below seem to have their own ventilation.

So most likely that pipe deserves some checking?